Authors a workstream brief for dispatching multiple parallel build agents across one codebase, with per-lane file ownership, shared-ground warnings, and a derived DO-NOT list. Use whenever work is about to be split across concurrent agents, workers, lanes, or waves — even if the user does not say "write a brief". Trigger phrases include "fan out", "parallel agents", "orchestrate this", "split into lanes", "workstreams", "run these in parallel", "review-gated build". Produces the brief; does not dispatch.
